This invention relates to a hydraulic brake booster having a booster piston forming on the side close to a brake pedal a boundary for a booster chamber and further including a valve spool which is slidable by means of the brake pedal to provide for alternate connection of the booster chamber with a source of pressure-transmitting fluid and with an unpressurized return reservoir, wherein the source of pressure-transmitting fluid is a pump-circulated fluid adapted to be throttled by a controlled flow-control valve, and wherein the booster piston is adapted to activate a static master cylinder via a push rod. Such a brake booster is known from the German laid open printed application DT-OS No. 2,434,236 published Feb. 6, 1975.
In the known brake booster, the flow-control valve is disposed in parallel with the booster piston, and a valve member of the flow-control valve is biassed in the closing direction by means of a spring. As a result of this bias, the pump-circulated fluid is continuously throttled so that there is at all times a low pressure head that can be metered into the booster chamber through the valve spool. In addition to the spring, the pressure in the booster chamber is adapted to act upon the valve member so that, during building up of pressure in the booster chamber, the valve member is urged into the closing direction to a greater degree, whereby the throttling of the pump-circulated fluid is augmented which results in a correspondingly increased pressure head. Thus, in the known brake booster, the pressure head generated lies always above the pressure in the booster chamber by the degree determined by the spring.
The disadvantage of this arrangement is that a continuous power loss has to be put up with even when the brake is not activated. It is to be considered also that the operating time of the vehicle is a multiple of the times in which the brake is activated and in which a pressure head is required. In addition to the continuous power loss, it is another disadvantage therein that the pressure fluid keeps being heated up because of the continuous throttling. This necessitates additional cooling installations in the pressure transmitting fluid circuit.